1. Field of the Invention
The present invention relates to a process for producing vinyl chloride resin, and more particularly to a process for producing vinyl chloride resin high in polymerization productivity, high in bulk density and with less fish eye by the use of a polymerization reactor equipped with a reflux condenser.
2. Description of the Prior Art
In the production of vinyl chloride resin it is often the case that a reflux condenser is used for improving productivity as well as for energy-saving but this is accompanied by problems that suspension polymerization under cooling by a reflux condenser often causes an increased porosity in the particle interior and affected smoothness of the particle's surface and the resulting deterioration of the filling property causes lowering of the bulk density and an increase of fish eye. As to bulk density, it is well known that it is correlated with vinyl chloride resin's productivity in processing and lowering of bulk density is known to cause lowering of extruder's output to thus result in aggravation of productivity in processing. As means of improving polyvinyl chloride's bulk density there is known, for instance, a method of adding vinyl chloride monomer in the course of polymerization (Japanese Laid-Open Pat. App. No. 97679/75) but the vinyl chloride resin obtained by this process is known to have many fish eyes and, moreover, the residual monomer in the resin is difficult to remove.
Meanwhile, the market's need for less fish eye of vinyl chloride resin (hereinafter referred to as "PVC") has been getting more severe year after year and general tendency is that fish eye of PVC plasticized with a high polymer plasticizer of relatively low plasticizing ability and high viscosity such as of polyester series is being taken up as problematic. In order to solve the problem of fish eye, it is recommended to prevent formation of low-porosity particles caused by low dispersion frequency through best possible improvement of dispersion-coalescence frequency of monomer droplets and also to improve the homogeneity of particles in the polymerized system through local monomer addition polymerization due to monomer condensed in the reflux condenser taking place in the top layer of polymer suspension through inhibition of bubbling in the middle stage of polymerization and thereafter. When a reflux condenser is used, gas generating from the monomer droplets are contained in the polymer suspension to result in lower homogeneity attainable by stirring and results in aggravation of fish eye through bubbling phenomenon in which polymer particles form a floating creamy layer on the polymer suspension in the middle stage of polymerization and thereafter.
Further, when the bubbling phenomenon is marked, there is caused another problem that the polymer suspension overflows into the reflux condenser and its piping to cause deposition of scales to adversely affect the product's quality, also causing lowering of the heat-removing capacity of the reflux condenser and seriously affecting the safety control of the producing process.